A. Field of the Invention
This invention relates generally to dialysis machines, including peritoneal and hemodialysis machines, and more particularly to the adjustment of the conductivity of a dialysate solution prepared by the machine so as to insure the proper conductivity of the solution during a dialysis session. The invention is particularly useful in conjunction with a dialysis machine that prepares and stores an entire batch of dialysate solution for the dialysis session, as opposed to dialysis machines based on so-called "proportioning" systems. As used herein, the term "batch" refers to the quantity of dialysate solution that is sufficient for one complete dialysis session for a single patient.
B. Statement of Related Art
Dialysis, including hemodialysis and peritoneal dialysis, is a treatment for patients that suffer from inadequate kidney function. In hemodialysis, blood is pumped from the patient's body through an extracorporeal artificial kidney circuit to a dialyzer. Bloodborne toxins and excess water are filled out of the blood through a semipermeable dialyzer membrane into an electrolyte (dialysate) solution. In peritoneal dialysis, the patient infuses a quantity of dialysate into the peritoneal cavity, and the peritoneal membrane acts as the semipermeable membrane. After a dwell period, the dialysate fluid is drained and a fresh supply of peritoneal dialysate is added to the peritoneal cavity.
Dialysate solution in hemodialysis and peritoneal dialysis machines has typically been prepared using a proportioning technique, wherein a concentrated dialysate solution is mixed with water to form a dialysate solution with the aid of a proportioning pump. To control the dialysate conductivity in a proportioning system, the proportioning pump is precisely regulated, or alternatively, the rate of supply of the dialysate powder and water to a mixing vessel is regulated. A representative dialysis machine that uses precise control of proportioning pumps is described in the Peterson et al. patent, U.S. Pat. No. 5,247,434.
In a batch system, unlike a proportioning system, an entire batch of dialysate (e.g., approximately 50 liters) is prepared shortly before the dialysis session and stored in a dialysate solution tank. In a batch system embodiment of the present invention, dialysate is prepared from a powdered sodium bicarbonate formulation and a liquid acid concentrate that are added to the dialysate solution tank and mixed to water to form an approximately 50 liter batch. The dialysate powder and liquid acid concentrates may be stored in individual containers during shipping, and are opened at the time of preparation of the solution and their contents added to the tank.
Since the total volume of the dialysate tank and associated dialysate fluid circuits is reasonably constant and capable of being determined experimentally, one method of insuring that the concentration and conductivity of the resulting dialysate solution is precisely control the quantity and formulation of the dialysate concentrates that are used to make up each batch of dialysate. This method would work if it were possible to also precisely control the total fluid volume of the dialysate fluid circuit including the dialysate tank.
In practice, the above method has disadvantages. First, precise control of the quantity and formulation of the dialysate concentrates in their individual vessels is difficult and expensive. Additionally, the total fluid volume of the dialysate circuit and tank can vary due to manufacturing tolerances and changes in components over the life cycle of the dialysis machine. If the tank and associated dialysate fluid circuit are subject to heat disinfection cycles, this can cause a slight expansion of the components resulting in an increase in system volume. It has been determined that even small fluctuations in total system volume can cause conductivity measurements that are out of a desired range, unless the quantity of the bicarbonate and liquid acid concentrates added during the preparation of the batch of dialysate are precisely controlled.
The present invention addresses these problems in a novel and unobvious way, and in the process avoids the need for maintaining precise control over (a) the quantities of chemicals in the dialysate vessel used to prepare the solution and (b) the total volume of the dialysis fluid circuit and tank. The invention provides a method of insuring that the conductivity of a batch of dialysate is within a predetermined desired conductivity range. Hence, an object of the invention is to provide a method for effectively controlling the dialysate concentration of a batch of dialysate, insuring that the dialysate concentration and conductivity is at a desired level prior to initiation of dialysis.